Signalling and localisation device for an individual in the sea, includingan ultrasonic wave transmitter constituting an individual alarm unit carried by the individual in the sea designed to transmit carrier waves of primary signals; a surface surveillance means arranged on the surface of the sea and provided with: a submersed antenna capable of picking up the primary signals transmitted by individual alarm unit; means of transmitting secondary ultrasonic signals, these transmission means: means capable of calculating the position of individual alarm unit as a function of the ultrasonic signals received in response to secondary ultrasonic signals and of converting this information into the coordinates of a global positioning system; a unit for transmitting radio waves capable of sending signals representing these coordinates to a mobile or fixed alarm processing centre equipped with a radio receiver.